Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 14 2024 00:25:14 FOUS30 KWBC 140025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 825 PM EDT Tue Aug 13 2024 Day 1 Valid 01Z Wed Aug 14 2024 - 12Z Wed Aug 14 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDDLE MISSISSIPPI VALLEY AND CENTRAL HIGH PLAINS... ....Portions of the Middle Mississippi Valley... While 1-3" of rain has fallen over the past 6-12 hours across=20 portions of eastern KS into southwestern MO, the greater flash=20 flood risk is likely to evolve late tonight into Wednesday morning=20 over the region. The ingredients remain favorable for a potentially significant training convective axis, and the latest hi-res CAMs=20 (18z HREF) continue to support this scenario. Robust instability=20 will be advecting in from the southwest, likely resulting in a=20 tight instability gradient somewhere in the vicinity of eastern KS=20 into western to central MO. Meanwhile, the low-level jet will=20 increase, resulting in strong and persistent moisture transport and convergence over the region. Corfidi vectors end up becoming quite weak, and what movement they do indicate is generally parallel to=20 the expected moisture convergence axis and instability gradient.=20 Thus we very well may end up with backbuilding upscale convective=20 growth over this area by Tuesday night into Wednesday morning. Confidence in there being flash flood impacts remains relatively high, although the axis of heaviest rainfall will likely be quite=20 narrow...thus there remains uncertainty on where exactly this ends=20 up. The HREF data continues to support a narrow axis of 3-5"+=20 rainfall, per 40-km neighborhood 5" exceedance probabilities of 30-50%. This 5" threshold is near the 6-hr Flash Flood guidance, which suggests a higher-end Slight risk (closer to a Moderate Risk than a low-end Slight Risk), with a narrow corridor of locally=20 significant impacts possible.=20 ....Portions of the Central High Plains... Have maintained (and shifted slightly eastward) a Slight Risk area over portions of northeast CO and adjacent areas of far southeast=20 WY, southwest NE, and northwest KS. Convection has proliferated in=20 the vicinity of a surface low pressure system over the higher=20 terrain of northern CO, quickly organizing and moving off the=20 terrain into the central High Plains (see MPD 878 for more info). Given the coverage and intensity of the convection, the risk of excessive rainfall is judged to be low-end Slight Risk.=20 ....Portions of the Southeast... Diurnal convection occurred once again over portions of GA/SC/NC today with surface low pressure positioned near the GA/SC border, resulting in localized 1-3" totals. Guidance (18z HREF) suggested additional 1-3" localized totals are possible, mainly in the next few hours, before activity gradually declines with the loss of daytime heating. The Slight Risk was was a removed and a Marginal Risk maintained to account for this localized risk.=20=20 ....Portions of the Southwest into Intermountain West... Scattered convection has commenced again from portions of AZ/NM=20 northward into ID/WY/MT. There has been a bit more in the way of=20 westerly flow compared to earlier days, which has allowed cells to=20 move along at a faster pace. This has kept the flash flood risk=20 rather isolated in nature, but heavy rainfall rates have been most problematic over southern UT. This activity should gradually wind down into the evening with loss of daytime heating.=20 Churchill/Chenard Day 2 Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDDLE AND UPPER MISSISSIPPI VALLEY... 2030Z Update... Convection should be on-going across portions of the Middle/Upper Mississippi Valley as Day 2 begins at 12Z on Wednesday...with model still showing the potential for training of cells given the alignment of low level winds. The area of rain expands during the latter portion of the period as surface pressures drop in response to the approach of a mid level shortwave trough and increasing low level warm advection. Spaghetti plots focus the possibility of 2 and 3 inch rainfall amounts in parts of the Mid- and Upper Mississippi Valley. Except for the need of a slight northward expansion in parts of the Upper Midwest...the previously issued excessive rainfall outlook had it covered well. Bann ....Previous Discussion... Training convection will likely be ongoing at 12z Wednesday across portions of MO. See the day 1 discussion for more details on this=20 expected evolution...but in summary the ingredients will be there=20 for a significant training threat to continue into Wednesday=20 morning. Just like on day 1, there remains some uncertainty on the=20 exact axis of maximum training, but this is likely a higher end=20 Slight risk at the moment, with some flash flooding likely wherever the axis sets up. By late morning into the afternoon the=20 convection should become more progressive and/or weaken as it=20 shifts southeastward. Thereafter the risk of excessive rainfall shifts northward into portions of IA and southern MN. Convection here will likely develop by afternoon and continue into the overnight hours, with the focus expected to be near and just north of a warm front slowly lifting north across the region. Stronger mid/upper forcing will move into the region, which combined with strengthening low level moisture transport, should allow for organized convective development. With the front not moving much and the moisture transport axis rather persistent, expect we will see some backbuilding/training convection near the front. Where exactly this sets up remains a bit of a question. The 00z GFS/NAM/UKMET have trended northward into more of MN, however the GEM reg, AIFS and GFS Graphcast all still support IA into far southern MN as the preferred location. With models often too far north with convective QPF, hesitant to expand the Slight risk too far north. Thus still think keeping it centered over IA and only into far southern MN is the way to go for now...generally favoring the the further south model solutions mentioned above. A secondary rainfall maximum may occur over portions of ND into far northern SD along another slow moving convergence axis. The 00z HREF data is supportive of Slight risk level impacts across this region. However HREF skill at these longer lead times is sometimes questionable, and global model solutions show a bit more spread on the convective evolution over this area. Given this uncertainty and the fact that this area is typically not as flash flood prone, decided to keep the risk level at Marginal for now and continue to monitor. Chenard Day 3 Valid 12Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF ILLINOIS, INDIANA AND SOUTHERN WISCONSIN... ....2030Z Update... The primary focus for heavy rainfall will shift eastward in association with the larger scale forcing. Deterministic QPF and ensembles still support the previously-issued Slight Risk area over portions of the Upper Lakes so only minor adjustments were made. Bann ....Previous Discussion... The heavy rainfall threat shifts east into the Great Lakes and OH Valley by Thursday. A bit more uncertainty with the details by this time, but the overall ingredients remain in place for areas of excessive rainfall. Expect one round of convection to be moving across the region Thursday morning, but with broad troughing moving through, this will likely not be the end of the convective risk. Instead we should be able to reload with instability and get additional round(s) of convection into Thursday night. Exactly where the best risk of multiple heavy convective rounds exists is still uncertain, but the most likely axis appears to be across portions of IL/IN. This is a bit south and east of the inherited Slight risk, and so the risk was adjusted accordingly. There is still some excessive rainfall threat further north over portions of WI and MI, but at the moment think the greater training risk is further south and east where the favorable ingredients should linger longer. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4xXzaEoGuhJe9dpVLr3agQRLsiwNx8Z47izLaprX9DBs= 7acX040xG3bHDjAgaKAJH8p87YcMkEelfZYk_7tshME3Zyc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4xXzaEoGuhJe9dpVLr3agQRLsiwNx8Z47izLaprX9DBs= 7acX040xG3bHDjAgaKAJH8p87YcMkEelfZYk_7ts5uixefI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4xXzaEoGuhJe9dpVLr3agQRLsiwNx8Z47izLaprX9DBs= 7acX040xG3bHDjAgaKAJH8p87YcMkEelfZYk_7tsPlbHlIs$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .